One also mustn't forget that there is a huge difference between such a store and a general provider like Dexsoft or 3DFoin. One thing is that there often is a hassle when importing e.g. a model and getting things like animations right (not all shops offer proper files that fit into every engine just out of the box). The other thing which most likely forms the major one is the engine ready content. You cannot buy yourself a particle system, pathfinding algorithm or editor extension anywhere else. Especially when thinking about Unity it's awesome what can be found in their store. I think any engine with a considerable amount of users just has to have such a store nowadays...